In recent years, attention has been given to technologies that produce bioethanol by fermentation, as technologies that can reduce the consumptions of oil resources and the amount of carbon dioxide emissions to produce fuel and industrial materials which are sustainable against the backdrop of increases in global environmental awareness over the whole world, steep rises in crude oil prices and the like.
However, increases in production of biofuel from food grain (for example, corn, tubers and roots, sugar cane or the like) have caused steep rises in food prices, and production of ethanol from cellulose-containing biomass (for example, herbaceous biomass such as bagasse, switchgrass, corn stover, rice straw, or wheat straw; or wood-based biomass such as trees or waste building materials) has become an important technological development.
As a method of producing alcohol from cellulose-containing biomass, a method of producing ethanol from cellulose-containing biomass is disclosed in a technological report (D. Humbird et al., “Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol”, NREL Report No. TP-5100-47764, May 2011) of NREL (National Renewable Energy Laboratory).
In the method disclosed in D. Humbird et al., “Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol”, NREL Report No. TP-5100-47764, May (2011), cellulose-containing biomass is first subjected to pretreatment using water vapor, acid or alkali (pretreatment step) to facilitate treatment of saccharification enzyme in a subsequent step. The cellulose-containing biomass subjected to the pretreatment is then supplied to a saccharification/fermentation tank, a saccharification enzyme (for example, cellulase or the like) and a nutrient are added to the saccharification/fermentation tank, which is then inoculated with ethanol fermentation bacteria to simultaneously perform saccharification and fermentation (saccharification step and fermentation step). As a result, a fermentation liquid is obtained. The fermentation liquid contains saccharification residual solids (most of which are lignin and the like), ethanol fermentation bacteria and the like. Thus, the obtained fermentation liquid is distilled to purify and recover ethanol (distillation step). The saccharification step and the fermentation step are simultaneously performed in the method, to therefore enable the number of necessary tanks to be reduced, and a capital investment can be therefore reduced so that the method is considered to be a competitive method.
In another method of producing alcohol from cellulose-containing biomass, an aqueous sugar solution obtained by hydrolyzing cellulose-containing biomass is filtered to remove a fermentation inhibitor contained in the aqueous sugar solution, followed by using the purified aqueous sugar solution to enable ethanol fermentation (WO 2010/067785).
In such a method of producing alcohol from cellulose-containing biomass as in D. Humbird et al., “Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol”, NREL Report No. TP-5100-47764, May (2011) mentioned above, the concentration of ethanol obtained in the fermentation step depends on the amount of cellulose-containing biomass that can be put in a saccharification/fermentation tank. Therefore, the amount of cellulose-containing biomass that can be supplied into the saccharification/fermentation tank precludes the concentration of ethanol obtained in the fermentation step from being, for example, 5.5 wt % or more so that a large amount of distillation residual liquid generated after a distillation step (for example, about 17 times greater than the amount of produced ethanol) is generated to cause the large load of treatment of discharging the distillation residual liquid.
In such a method of producing alcohol from cellulose-containing biomass as in WO 2010/067785, an ethanol fermentation liquid can be obtained from an aqueous sugar solution. However, further improvement of treatment of a distillation residual liquid generated when an obtained ethanol fermentation liquid is distilled to recover ethanol is also required to further improve a method of producing ethanol.
In other words, a method of producing alcohol with the reduced amount of liquid discharged when alcohol is produced from cellulose-containing biomass is demanded, and a problem is to reduce the load of treatment of liquid discharged when a distillation residual liquid is treated.
Thus, it could be helpful to provide a method of producing alcohol from cellulose-containing biomass in which the amount of discharged liquid necessary to treat the discharged liquid when alcohol is produced from cellulose-containing biomass can be greatly reduced.